In last week’s update, we discussed how sanitation following spring melons was important in preventing whitefly buildups during the summer. Now that the melons are finished and growers are preparing ground for fall produce crops, another sanitation practice (weed management) is very important in preventing insect buildups in fall crops. Effective weed management is critical for the profitable production of vegetable crops in the desert southwest for all the obvious reasons. However, weed management is also essential for another important, but often overlooked, reason. Several common weed species found in and around vegetable crops can serve as host plants to many insect pests that can later infest nearby crops. Although flowering weeds can provide a reservoir for natural enemies, and a source of nectar and pollen for a pollinators, these same weedy refuges can serve as host sources for many key insect pests that cause economic damage to vegetable crops. Weeds found on field margins and ditch banks can provide insect pests with suitable resources needed for rapid population growth which subsequently can lead to insect infestations occurring in adjacent vegetable crops. In addition, many weed species can provide insects with host plants that serve as a bridge between cropping seasons when vegetables crops are not in production (i.e., July-August). Since most of these key insect pests have the ability to move relatively long distances to find new food sources, weeds that are allowed to grow unchecked in fallow fields during the summer often serve as a key source of insect infestations for fall vegetable and melon plantings. For example, pale-striped flea beetle and beet armyworm populations will commonly develop on common purslane that has been allowed to grow in fallow fields prior to fall vegetable and melon plantings. Volunteer melons and cotton can also be considered weeds (“a plant out of place”). If not controlled in a timely manner, these weedy volunteer plants can sustain large numbers of insect pests, as well as many of the plant viruses that are transmitted by insect vectors, that can migrate onto newly planted fields. Finally, weeds can serve as impediments to insecticide applications. Dense weed foliage in vegetable and melon fields can negatively influence foliar spray applications by intercepting spray droplets before reaching the target crop, which can result in less insecticide deposition and unacceptable crop damage. Soil applied insecticides (e.g., imidacloprid) can also be impacted by unmanaged weed growth. Weeds growing unchecked during stand establishment can compete with the seedling plants for water and fertilizer, but they can also compete with crop plants for soil insecticides. Excessive weed densities can significantly intercept insecticides in the soil profile and reduce the amount available for uptake by the target crop. For more information on this topic, please visit this report: Interactions between Insects and Weeds in Vegetable Crops.

Remember: “ When in Doubt…… Scout”

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To contact John Palumbo go to: jpalumbo@ag.arizona.edu

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.